202 Mr. H. C. Sorby on some Spectra of Compounds [Feb. 10, 



all of which occur at the red end, where no trace of bands exists in the case 

 of ordinary salts, as will be seen on comparing it with Nos. 1, 2, and 3. 

 I have tried many experiments in order to ascertain whether any other 

 element besides zirconia will cause uranium to give similar abnormal spectra, 

 but none show anything of the kind, at all events in similar conditions. 

 A few have special characters, as described below, but the majority exert 

 little or no influence ; and even when the blowpipe-beads are crystalline, 

 they show only the usual spectra of the oxides of uranium. Moreover no 

 such great change in the character of the spectra of any other elements 

 which give absorption-bands is to be seen when they are combined with 

 zirconia ; and, as far as my present experience goes, it seems as if such very 

 abnormal spectra were met with only in the case of these remarkable 

 compounds of zirconia with the oxides of uranium. 



Such, then, being the facts, it appears to me that we are now in a 

 position to explain why certain zircons give three different spectra, as 

 described in my former paper. Some jargons (usually those of a green 

 tint) contain a little uranium so combined that the characteristic spectrum 

 is only faintly visible, whereas, after ignition, the intensity of the absorp- 

 tion-bands is permanently increased to a variable extent, occasionally only 

 a little, but in some cases as much as twenty-five times. This more powerful 

 action on light is accompanied by an increase in hardness and in specific 

 gravity (sometimes as much as from 4*20 to 4*60), as described in my 

 former paper ; and I have since found that these changes are approximately 

 proportional to the amount of uranic oxide in the various specimens, as 

 shown by comparing the spectra of the blowpipe-beads. This change 

 may partly depend on the oxidization of the uranous oxide, since some 

 specimens slightly increase in weight when ignited ; but I think it cannot 

 be mainly due to that ; for sometimes there is no such increase, and uranous 

 oxide combined with zirconia gives rise, not to a spectrum without bands, 

 but to one with several of very marked character, as described below. On 

 the whole, since this abnormal type of spectrum is so characteristic of 

 combination with zirconia, it appears to me more probable that the effect 

 of a high temperature is to cause the uranic oxide to combine more specially 

 with the zirconia, as though the greater part existed naturally as a silicate, 

 but after ignition as a zirconiate. We may also apply the same explana- 

 tion in the case of zircons more or less strongly coloured by other oxides, 

 which become almost colourless when heated, and thus this unexplained 

 peculiarity of zircons may depend on the fact of zirconia being able to play 

 the part of both a base and an acid, which, as compared with silica, has 

 an affinity for bases varying according to the temperature. 



The brown-red zircon from Ceylon, named at page 514 of my former 

 paper, kindly presented to me by Mr. E. L. Mitford, of Rusthall, gives a 

 spectrum precisely like that of the borax blowpipe-beads crystallized after 

 treatment in the deoxidizing flame, and therefore no doubt contains uranous 

 oxide. This spectrum being given by only one part of the crystal, probably 



